Sugden A 41 amp noise with high frequences

I found two years ago a Sugden A41 amp and preamp in not perfect shape. I drove 2 hrs in a snow storm to pick up it. On the amp section, on the side, a resistor was dead and another had a bad spec, some point with heating trace on the board. The amp sounds great, with a very large soundstage and strong dynamic.

I am searching a diagnostic for a curious issue. There is a noise with high frequencies only, as a saturation noise. It is happening only with classical music especially early music. With Bob Marly or Queens no problem! Maybe the CD mastering removes high frequencies? Early music, is all about high pitch voice, children, men and women, luth and violin. When the amp is warmed up the noise seems to disappear or at least reduce.

It happens only on the right channel, of course, I inversed channel, cables, source ... to control.